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Xerographic Xerographic CopiersCopiers

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Introductory QuestionIntroductory Question

You are covered with static You are covered with static electricity. If you hold a sharp pin in electricity. If you hold a sharp pin in your hand as you reach out for a your hand as you reach out for a doorknob, the pin willdoorknob, the pin will

A.A. make the shock you’ll get worsemake the shock you’ll get worse

B.B. make the shock you’ll get weakermake the shock you’ll get weaker

C.C. leave the shock you’ll get unchangedleave the shock you’ll get unchanged

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Observations About Observations About CopiersCopiers

Copies consist of black powder stuck on Copies consist of black powder stuck on paperpaper

After jams, the powder sometimes wipes After jams, the powder sometimes wipes offoff

Copiers run out of black powder or “toner “Copiers run out of black powder or “toner “ Copies are often warm after being madeCopies are often warm after being made Copies are staticy and clingyCopies are staticy and clingy Some copiers scan a light, some use a flashSome copiers scan a light, some use a flash

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3 Questions about3 Questions aboutXerographic CopiersXerographic Copiers

How can light arrange black powder How can light arrange black powder on paper?on paper?

How does a copier spray charge on a How does a copier spray charge on a surface?surface?

How does a copier make its copies How does a copier make its copies permanent?permanent?

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Question 1Question 1

How can light arrange black powder How can light arrange black powder on paper?on paper?

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The Xerographic ConceptThe Xerographic Concept The xerographic conceptThe xerographic concept

projects an image of the document onto a surfaceprojects an image of the document onto a surface that has been sprayed with electric charges.that has been sprayed with electric charges. Wherever light hits that surface, the charges Wherever light hits that surface, the charges

escape.escape. The remaining charges attracts black toner The remaining charges attracts black toner

particlesparticles which are then attached to paper to produce a which are then attached to paper to produce a

copy.copy. The surface is a photoconductor,The surface is a photoconductor,

an insulator that turns into a conductor in the light,an insulator that turns into a conductor in the light, so illumination allows charges to move and escape.so illumination allows charges to move and escape.

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Question 2Question 2

How does a copier spray charge on a How does a copier spray charge on a surface?surface?

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Electric Fields Electric Fields (Part 1)(Part 1)

There are two ways to view electric forcesThere are two ways to view electric forces The first view is charge-on-charge,The first view is charge-on-charge,

in which charge 1 pushes directly on charge 2.in which charge 1 pushes directly on charge 2. The second view is charge-field-charge,The second view is charge-field-charge,

in which charge 1 creates an electric fieldin which charge 1 creates an electric field and that electric field pushes on charge 2.and that electric field pushes on charge 2.

Although this electric field sounds fictional,Although this electric field sounds fictional, it actually exists!it actually exists!

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Electric Fields Electric Fields (Part 1)(Part 1)

An electric fieldAn electric field is a structure in space that pushes on is a structure in space that pushes on

electric chargeelectric charge and is vector in character: magnitude and and is vector in character: magnitude and

direction.direction. At each point in space, the electric At each point in space, the electric

field’sfield’s magnitude is proportional to force on + magnitude is proportional to force on +

test chargetest charge and direction is direction of that force.and direction is direction of that force.

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Voltage Gradients andVoltage Gradients andElectric FieldsElectric Fields

Objects always accelerate so as to reduce Objects always accelerate so as to reduce their total potential energies as quickly as their total potential energies as quickly as possiblepossible

The total potential energy of a positive chargeThe total potential energy of a positive charge is proportional to the voltage at its location in is proportional to the voltage at its location in

space,space, so it accelerates toward decreasing voltage.so it accelerates toward decreasing voltage.

But a charge accelerates with an electric field,But a charge accelerates with an electric field, so a voltage gradient (a slope) so a voltage gradient (a slope) isis an electric an electric

field!field!

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Electric FieldsElectric FieldsIn and Around MetalsIn and Around Metals

Inside a metal, charges can move freely, soInside a metal, charges can move freely, so they can move to minimize their potential energies.they can move to minimize their potential energies. At equilibrium, the metal has a uniform voltage,At equilibrium, the metal has a uniform voltage, and there is no electric field inside the metal.and there is no electric field inside the metal.

Outside a metal, charges can’t move freely, soOutside a metal, charges can’t move freely, so they cannot minimize their potential energies.they cannot minimize their potential energies. At equilibrium, voltages can vary with location,At equilibrium, voltages can vary with location, and there can be an electric field outside the metal.and there can be an electric field outside the metal.

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Corona DischargesCorona Discharges

Outside a sharp or narrow metal at high Outside a sharp or narrow metal at high voltage,voltage, the voltage varies rapidly with position,the voltage varies rapidly with position, so that the electric field is very strongso that the electric field is very strong and it can push charges onto passing air and it can push charges onto passing air

particles.particles. This phenomenon is a corona dischargeThis phenomenon is a corona discharge

in which the narrow metal sprays chargesin which the narrow metal sprays charges and can easily deposit or dissipate static and can easily deposit or dissipate static

electricity.electricity.

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Introductory Question Introductory Question (revisited)(revisited)

You are covered with static You are covered with static electricity. If you hold a sharp pin in electricity. If you hold a sharp pin in your hand as you reach out for a your hand as you reach out for a doorknob, the pin willdoorknob, the pin will

A.A. make the shock you’ll get worsemake the shock you’ll get worse

B.B. make the shock you’ll get weakermake the shock you’ll get weaker

C.C. leave the shock you’ll get unchangedleave the shock you’ll get unchanged

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Question 3Question 3

How does a copier make its copies How does a copier make its copies permanent?permanent?

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Xerographic ProcessXerographic Process

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Copier StructureCopier Structure

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Summary aboutSummary aboutXerographic CopiersXerographic Copiers

It sprays charges from a corona dischargeIt sprays charges from a corona discharge Those charges precoat a photoconductorThose charges precoat a photoconductor It projects a light onto the photoconductorIt projects a light onto the photoconductor The charges escape from illuminated The charges escape from illuminated

regionsregions The remaining charges attract toner The remaining charges attract toner

particlesparticles Those particles are fused to the paper as a Those particles are fused to the paper as a

copycopy